Electrochemical synthesis and characterization of mixed molybdenum-rhenium oxides

The electrodeposition of Mo_xRe_1−xO_y films (0.6 ≤ x ≤ 1) on indium-tin oxide (ITO) coated glass substrates from acidic peroxo-polymolybdo-perrhenate solutions is described. Trends in film growth were established as a function of potential from +0.4 V to −0.7 V vs Ag/AgCl by analyzing the composition and stoichiometry of the deposit using inductively coupled plasma mass spectrometry (ICPMS) and X-ray photoelectron spectroscopy (XPS). These experiments show that the concentration of rhenium increases linearly with the deposition potential and that the deposits are mixed-valent containing up to five different metal oxidation states (i.e., Mo^IV, Mo^V, Mo^VI, Re⁰, Re^IV). Electroanalytical techniques were used to explore the deposition mechanism, including chronocoulometry, cyclic voltammetry, spectroelectrochemistry, and electrochemical quartz crystal nanogravimetry (EQCN). At potentials positive to −0.26 V, perrhenate (Re^VIIO₄⁻) behaves as a redox mediator to accelerate the deposition of a mixed-valent molybdenum oxide, but at more negative potentials mixed molybdenum-rhenium oxides are produced.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

Fabrication and electrical properties of MoS2 nanodisc-based back-gated field effect transistors

Two-dimensional (2D) molybdenum disulfide (MoS₂) is an attractive alternative semiconductor material for next-generation low-power nanoelectronic applications, due to its special structure and large bandgap. Here, we report the fabrication of large-area MoS₂ nanodiscs and their incorporation into back-gated field effect transistors (FETs) whose electrical properties we characterize. The MoS₂ nanodiscs, fabricated via chemical vapor deposition (CVD), are homogeneous and continuous, and their thickness of around 5 nm is equal to a few layers of MoS₂. In addition, we find that the MoS₂ nanodisc-based back-gated field effect transistors with nickel electrodes achieve very high performance. The transistors exhibit an on/off current ratio of up to 1.9 × 10⁵, and a maximum transconductance of up to 27 μS (5.4 μS/μm). Moreover, their mobility is as high as 368 cm²/Vs. Furthermore, the transistors have good output characteristics and can be easily modulated by the back gate. The electrical properties of the MoS₂ nanodisc transistors are better than or comparable to those values extracted from single and multilayer MoS₂ FETs.     

Molybdenum carbide embedded in carbon nanofiber as a 3D flexible anode with superior stability and high-rate performance for Li-ion batteries

The fabrication process and material design of flexible lithium-ion batteries (LIBs) are essential in flexible portable devices. In particular, the carbon nanofiber (CNF)-based active anodes with flexibility synthesized using an electrospinning technique showed fairly stable cycling performance in the LIBs. In this study, we synthesized the molybdenum carbide (MoC) embedded in CNFs as an anode for LIBs (MoC/CNF) using an electrospinning technique with amorphous Mo precursor and polyacrylonitrile as the molybdenum and carbon sources, respectively, and using a heating process under an N₂ atmosphere. The as-prepared flexible MoC/CNF showed a 3D porous structure consisting of crystalline MoC and amorphous CNF. MoC/CNF, directly utilized as an active electrode without binder, conductor, or current collector, exhibited superior LIB performance, i.e. high capacity, cyclability, and high-rate properties. In particular, at a considerably high charge/discharge rate of 10?A?g^?1, the specific capacity of MoC/CNF (109?mAh?g^?1) was significantly higher than that of pure CNF electrode (3?mAh?g^?1).     

Corrosion behavior of a Mo(Si,Al)2 composite at 1700°C in 95% N2 + 5% H2

A Mo(Si, Al)₂ based composite was pre-oxidized to establish an alumina scale on the material surface. Thereafter, the corrosion behavior of the composite was examined at 1700 °C for up to 24 h in 95% N₂ + 5% H₂.The weight change was followed by recording the material weight before and after exposure. The crystalline corrosion products were analyzed with X-ray diffraction (XRD) and the microstructure of the cross sectioned material was characterized using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS).It was shown that AlN and Al₅O₆N layers developed on top of the pre-oxidized alumina layer and alumina threads develop out from the specimen surface. The accompanied aluminum consumption converts the substrate Mo(Si,Al)₂ into Mo₅Si₃ immediately below the alumina scale to the extent that the Mo₅Si₃ becomes porous underneath the alumina scale. Corrosion mechanisms are discussed with the support of thermodynamic calculations.     

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties

The mechanical behavior of ZrB₂-MoSi₂ ceramics made of ZrB₂ powder with three different particle sizes and MoSi₂ additions from 5 to 70 vol% was characterized up to 1500 °C. Microhardness (12–17 GPa), Young’s modulus (450–540 GPa) and shear modulus (190–240 GPa) decreased with both increasing MoSi₂ content and with decreasing ZrB₂ grain size. Room temperature fracture toughness was unaffected by grain size or silicide content, whilst at 1500 °C in air it increased with MoSi₂ and ZrB₂ grain size, from 4.1 to 8.7 MPa m^½. Room temperature strength did not trend with MoSi₂ content, but increased with decreasing ZrB₂ grain size from 440 to 590 MPa for the largest starting particle size to 700–800 MPa for the finest due to the decreasing size of surface grain pullout. At 1500 °C, flexure strength for ZrB₂ with MoSi₂ contents above 25 vol% were roughly constant, 400–450 MPa, whilst for lower content strength was controlled by oxidation damages. Strength for compositions made using fine and medium ZrB₂ powders increased with increasing MoSi₂ content, 250–450 MPa. Ceramics made with coarse ZrB₂ displayed the highest strengths, which decreased with increasing MoSi₂ content from 600 to 450 MPa.     

[H3NNH3]2K2MnMo9O32·(NH3)·3H2O: synthesis and crystal structure determination of a novel heteropolyoxometallate related to the Waugh structure

A novel heteropolyoxometallate with the formula [H₃NNH₃]₂K₂MnMo₉O₃₂·(NH₃)·3H₂O has been prepared in water solution. The crystal structure of the title compound was determined by the single-crystal method. This new mixed metal anionic complex crystallizes in the rhombohedral space group R3 (No. 146) with a=b=15.873(5), c=12.344(6) Å, γ=120°. The disordered structure contains three molecules in the unit cell. Full-matrix least-squares refinements of the title complex yielded final reliability (R) factors of 0.0356 (Rw=0.0868) with a goodness-of-fit (GOF, Σ₂) value of 1.099 based on 1430 [I>2σ(I)] observed reflections. All Mo and Mn atoms are six-coordinated octahedrally to form a Mn-centered cage with a Mo₉O₃₂ group as the framework.     

Molybdenum disilicide-silicon nitride bushing nozzles tailor-made for basalt fibers production

A carefully comparative study of several ceramics compounds allowed the identification of MoSi₂ blended with Si₃N₄ as a suitable material for the realization of bushing nozzles, specifically designed for basalt fibers production. Degradation static tests were developed and performed to demonstrate the high resistance to oxidation at operating temperature of MoSi₂-Si₃N₄ composites with respect to alumina, zirconia stabilized with magnesia or yttria, pure MoSi₂, pure Si₃N₄, and MoSi₂ englobing SiC. Moreover, the MoSi₂-Si₃N₄ composites resulted resistant to corrosion and chemical inert with respect to oxidizing components present in basalt melts.Bushings in MoSi₂-Si₃N₄ were obtained by using hot isostatic pressing and were processed through mechanical drilling. The basalt fibers produced with the MoSi₂-Si₃N₄ bushing showed a constant diameter and a regular surface morphology. The bushing was recovered intact without defects or fractures demonstrating the efficiency of the selected material. Replacing platinum-rhodium bushings with ceramic ones could greatly reduce costs of basaltic fibers production and could encourage the use of these eco-friendly, natural fibers as reinforcement in lightweight polymer composites.     

Sulfided Mo and CoMo supported on zeolite as hydrodesulfurization catalysts: transformation of dibenzothiophene and 4,6-dimethyldibenzothiophene

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out on sulfided Mo and CoMo on HY catalysts, and also on sulfided Mo and CoMo on alumina catalysts (fixed bed reactor, 330°C, 3 MPa hydrogen pressure). On all the catalysts, the two reactants transformed through the same parallel pathways: direct desulfurization (DDS) leading to biphenyl-type compounds, and desulfurization after hydrogenation (HYD) leading first to tetrahydrogenated intermediates, then to cyclohexylbenzene-type products. However, additional reactions were observed with the zeolite-supported catalysts, namely methylation of the reactants, cracking of the desulfurized products, and, in the case of 4,6-DMDBT, displacement of the methyl groups and transalkylation. The global activity of Mo/zeolite in DBT or 4,6-DMDBT transformation as well as its activity for the production of desulfurized products (HDS) were much higher than those of Mo/alumina. On the other hand, cobalt exerted a promoting effect on the activity in the transformation of DBT or 4,6-DMDBT of all the molybdenum catalysts. However, this effect was much less significant with the zeolite support than with the alumina support, which indicated that the promoter was not well associated to molybdenum on the zeolite support. Therefore, the activity of CoMo/zeolite in the HDS of DBT was much lower than that of CoMo/alumina. On the contrary, in the case of 4,6-DMDBT CoMo/zeolite was more active in HDS than CoMo/alumina. This increase in HDS activity was attributed to the transformation of 4,6-DMDBT into more reactive isomers through an acid-catalyzed methyl migration. The consequence was that on the zeolite-supported catalyst 4,6-DMDBT was more reactive than DBT.     

Removal of molybdenum using silver nanoparticles from water samples: Particle swarm optimization–artificial neural network

In this study, a simple and fast method for preconcentration and determination of trace amount of molybdenum from water samples was developed by silver nanoparticles based solid-phase extraction method and UV–vis spectrophotometry. Hybrid of artificial neural network–particle swarm optimization (ANN–PSO) has been used to develop predictive models for simulation and optimization of solid phase extraction method. Under the optimum conditions, the detection limit and relative standard deviation were 11 μg L⁻¹ and <3.9%, respectively. The pre-concentration factor of this method was 50. The method was applied to preconcentration and determination of molybdenum from water samples.